Electrical coupling and/or gap junctions (the structures generally responsible for coupling) have been observed in every metazoan embryo that has been examined; although such structures were first described 20 years ago, their role in embryonic development remains unknown. The experiments in this proposal will use an in vitro model to identify and characterize agents that regulate the formation of gap junctions in peripheral neural tissue. This laboratory has found that fetal rat sympathetic neurons can be maintained in vitro under conditions in which virtually all or virtually none of the neurons are electrically coupled; this is the first neural population in which it has been possible to chronically manipulate electrical coupling. Initially experiments will be done to identify agents capable of inducing electrical coupling among neurons. Subsequent experiments will extensively characterize the mechanism of action and the chemical nature of a crude material obtained from fetal calf serum that acts chronically to decrease coupling among sympathetic neurons in vitro; if this material acts in vivo as it does in vitro, it will provide a powerful tool for analyzing the developmental role of gap junctions. After agents capable of chronically altering junctional communication among sympathetic neurons have been characterized, a comparison will be made of the regulation of coupling in vitro among cells that are coupled in situ (ciliary neurons) and cells that are not coupled in situ (sympathetic neurons). Such a comparison will determine whether agents that alter coupling among sympathetic neurons have a general action upon other peripheral neural tissues; it may also be helpful in determining why only a subpopulation of neural tissue is coupled in adult animals. The identification of agents that chronically alter coupling among fetal neurons is necessary to allow future investigations that examine the types of developmental information that can be conveyed across gap junctions. Thus, this study of the regulation of electrical coupling in fetal neural tissue represents initial progress towards defining the role of gap junctions in neural development.